The researchers bred male or sire crickets with female or dam siblings, then examined the offspring for sperm viability and three measures of ability to fight off or resist infection.

On two out of the three measures, crickets with better quality sperm were better at fighting off infection.

But on the third key measure, crickets with relatively high sperm viability had relatively low levels of the enzyme lysozyme, the crickets' primary means of fighting bacterial infection.

This finding supports the hypothesis that the trade-off is part of the way that a male's sexual traits indicate to females whether their potential offspring are likely to be resistant to disease, the reseachers say.

Reproductive success

Australian evolutionary biologist Dr Rob Brooks, who was not involved in the research, says the findings are important because it confirms theory.

"Mathematical theory about how evolution works has been predicting this kind of result for some time now, so it's very interesting to see that theory confirmed in a real organism," says Brooks, from the University of New South Wales in Sydney.

"But the real value of the research goes beyond the field cricket; it goes to understanding reproductive success in all sorts of organisms," he says.

"The authors mention in the study that ... men that are treated with antibiotics or drugs that suppress the immune system have increased fertility and increased sperm quality, so we think this is a general principle that shapes the relationship between sperm and the immune system."

The researchers say that previous studies show that fish also have a trade-off between sperm quality and immunity.